1. Field of the Invention
This invention relates to a group of compounds which are useful for stabilizing the dyes in an electrophoretic display system.
2. Brief Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing the migration of charged pigment particles in a solvent, preferably a colored dielectric solvent. This type of display was first proposed in 1969. An EPD typically comprises a pair of opposed and spaced-apart plate-like electrodes, with spacers predetermining a certain distance between the electrodes. At least one of the electrodes, typically on the viewing side, is transparent. For the passive type of EPDs, row and column electrodes on the top (the viewing side) and bottom plates respectively, are needed to drive the displays. In contrast, an array of thin film transistors (TFTs) on the bottom plate and a common, non-patterned transparent conductor plate on the top viewing substrate are required for the active type EPDs.
An electrophoretic fluid composed of a colored dielectric solvent or solvent mixture and charged pigment particles dispersed therein is enclosed between the two electrode plates. When a voltage difference is imposed between the two electrode plates, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the plates, can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages or pulsing time.
EPDs of different pixel or cell structures have been reported previously, for example, the partition-type EPD (M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148–1152 (1979)) and the microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026).
An improved EPD technology was disclosed in co-pending applications, U.S. Ser. No. 09/518,488 filed on Mar. 3, 2000 (corresponding to WO 01/67170), U.S. Ser. No. 09/606,654 filed on Jun. 28, 2000 (corresponding to WO 02/01281) and U.S. Ser. No. 09/784,972 filed on Feb. 15, 2001 (corresponding to WO02/65215), all of which are incorporated herein by reference. The improved EPD comprises isolated cells formed from microcups and filled with charged particles dispersed in a dielectric solvent. The filled cells are individually sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from a group consisting of thermoplastics, thermosets and precursors thereof.
For any type of the electrophoretic displays, the electrophoretic fluid contained within the display cells is undoubtedly one of the most crucial parts of the device. The fluid, as stated earlier, usually is composed of pigment particles dispersed in a dielectric solvent or solvent mixture. Halogenated solvents of a high specific gravity have been widely used as the dielectric solvent in EPD applications, particularly those involving an inorganic pigment, such as TiO2, as the charged particles. The halogenated solvents of a high specific gravity are very useful in reducing the rate of sedimentation of the pigment particles in the solvent. Fluorinated solvents are among the most preferred because they are chemically stable and environmentally friendly.
The display fluid may be colored by dissolving or dispersing a dye or colorant in the dielectric solvent or solvent mixture. However, the color of the dye or colorant may fade due to thermal and/or photooxidation. To remedy this problem, a stabilizer or quencher is often added to the fluid to inhibit or suppress photooxidation of the dye or colorant through quenching the excited state and in some cases, quenching the radicals present in the system. Radical quenchers or inhibitors include phenols, oximes, TEMPO, nitroso compounds or derivatives and metal complexes thereof. Metal complexes are of particular interest because of their high efficiency in quenching the excited state and inhibiting the photooxidation process. Unfortunately, most quenchers are not soluble in halogenated dielectric solvents, particularly not in fluorinated solvents.